Mathematical and computer models are used to predict fluxes of amino acids into and out of cells. The predictions are compared with results of experimental analysis of ascites cells and frog muscle. Results to date suggest that the bulk of the amino acids are free in the cell water and not bound to cytoplasmic sites. C13 nuclear magnetic resonance will be used to check this conclusion. The data also suggest that amino acids, although accumulated against a concentration gradient at 0 degrees Centigrade in frog muscle are confined to a compartment within the muscle. Compartmental analysis of sugar and amino acid accumulation will be used to find out whether this compartment is the sarcoplasmic reticulum. NMR studies of the water protons in the frog muscle system may also shed some light on this problem. Studies of the amino acid accumulation ability of the microsomal fraction of frog muscle will be initiated. Studies on the interaction of monovalent cation with amino acid transport in ascites cells will be continued.